The mmap(2) flag indicate that the mapping is used as a stack.

https://man.openbsd.org/mmap.2#MAP_STACK
https://marc.info/?l=openbsd-tech&m=152035796722258&w=2

Was able to move some of them up one level so they are valid on more
platforms with less duplication

CI still runs version 10

Available since FreeBSD 9.0, this API allows using local descriptors
instead of global PIDs. It works in Capsicum sandboxes.

Apparently 17.04 is broken now?

Useful when playing around with mounting points, sadly not a common
interface

Note that OpenBSD and OSX do not support POSIX message queues.